Host-pathogen interactions in plants. Plants, when exposed to oligosaccharides of fungal origin, defend themselves by accumulating antibiotics

Plants are exposed to attack by an immense array of microorganisms, yet are resistant to almost all of these potential pests. Many plants respond to an invasion by a pathogenic or nonpathogenic microorganism, whether a fungus, a bacterium or a virus, by accumulating phytoalexins, low molecular weight compounds which inhibit the growth of microorganisms. Phytoalexins are probably also toxic to higher animal and plant cells (38). The production of phytoalexins appears to be a widespread mechanism by which plants attempt to defend themselves against microbes and, perhaps, against other pests (6, 16, 17, 28, 32, 36, 37, 38, 63). Molecules of microbial origin which trigger phytoalexin accumulation in plants have been called elicitors (33). Plants recognize and respond to elicitors as foreign molecules. However, plants are unlikely to have sufficient genetic material to code for unique recognition systems for every bacterial species and strain and every fungal race and virus that plants are exposed to and respond to defensively. Thus, elicitors are likely to be molecules present in many different microbes and, in fact, the elicitor to be described in this paper is a structural polysaccharide of the mycelial walls of many fungi (11).